本文介绍利用FFmpeg在iOS上将H.264与AAC封装成MP4文件，从而实现直播中本地录像的功能。

写MP4文件只需三步：
1.写文件头
2.填充数据
3.写文件尾

用到的api大致如下：
1.avformat_alloc_output_context2()；
2.avformat_new_stream();
2.avio_open();
3.avformat_write_header()；
4.av_interleaved_write_frame();
5.av_write_trailer();

下面是示例代码（此代码基于kxmovie源码）
1.写头：

- (void)startRecordAction {
    //在沙盒中创建用于保存mp4的文件
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory,NSUserDomainMask,YES);
    NSString *homePath = [paths objectAtIndex:0];
    NSCalendar *curCalendar = [NSCalendar currentCalendar];
    NSUInteger unitFlags = NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay | NSCalendarUnitHour | NSCalendarUnitMinute | NSCalendarUnitSecond;
    NSDateComponents *dateComponents = [curCalendar components:unitFlags fromDate:[NSDate date]];
    filePath = [homePath stringByAppendingPathComponent:[NSString stringWithFormat:@"%ld%02ld%02ld%02ld%02ld%02ld.mp4",(long)dateComponents.year,(long)dateComponents.month,(long)dateComponents.day,(long)dateComponents.hour, (long)dateComponents.minute, (long)dateComponents.second]];
    const char *filename = [filePath cStringUsingEncoding:NSASCIIStringEncoding];
    // find first video stream
    if (!_formatCtx) {
        return;
    }
    for (unsigned i=0; i<_formatCtx->nb_streams; i++) {
        if (_formatCtx->streams[i]->codec->codec_type == AVMEDIA_TYPE_VIDEO) {
            i_video_stream = _formatCtx->streams[i];
        }
        else if (_formatCtx->streams[i]->codec->codec_type == AVMEDIA_TYPE_AUDIO) {
            i_audio_stream = _formatCtx->streams[i];
        }
    }
    if (i_video_stream == NULL) {
        return;
    }
    else if (i_audio_stream == NULL) {
        _isNoAudio = YES;
    }
    if (i_video_stream->codec->codec_id == AV_CODEC_ID_HEVC) {
        _isStartWriteFile = NO;
        return;
    }
    
    avformat_alloc_output_context2(&o_fmt_ctx, NULL, "mp4", filename);//初始化一个默认的AVFormatContext,用于输出文件
    //设置参数
    o_video_stream = avformat_new_stream(o_fmt_ctx, NULL);
    AVCodecContext *v;
    AVCodecContext *i;
    v = o_video_stream->codec;
    i = i_video_stream->codec;
    v->bit_rate = 400000;
    v->codec_id = i->codec_id;
    v->codec_type = i->codec_type;
    v->time_base.num = i->time_base.num;
    v->time_base.den = i->time_base.den;
    o_video_stream->time_base.num = i_video_stream->time_base.num;
    o_video_stream->time_base.den = i_video_stream->time_base.den;
    v->width = i->width;
    v->height = i->height;
    v->pix_fmt = i->pix_fmt;
    v->flags = i->flags;
    v->flags |= CODEC_FLAG_GLOBAL_HEADER;
    v->extradata = i->extradata;
    v->extradata_size = i->extradata_size;
    if (!_isNoAudio) {
        //音频参数
        o_audio_stream = avformat_new_stream(o_fmt_ctx, NULL);
        AVCodecContext *a;
        AVCodecContext *i;
        a = o_audio_stream->codec;
        i = i_audio_stream->codec;
        a->codec_id = AV_CODEC_ID_AAC;
        a->codec_type = AVMEDIA_TYPE_AUDIO;
        a->channel_layout = i->channel_layout;
        a->audio_service_type = i->audio_service_type;
        a->sample_rate = i->sample_rate;
        a->channels = i->channels;
        a->sample_fmt = i->sample_fmt;
        a->bit_rate = i->bit_rate;
        a->flags = i->flags;
        a->flags |= CODEC_FLAG_GLOBAL_HEADER;
        a->time_base.num = i->time_base.num;
        a->time_base.den = i->time_base.den;
        o_audio_stream->time_base.num = i_audio_stream->time_base.num;
        o_audio_stream->time_base.den = i_audio_stream->time_base.den;
        a->extradata = i->extradata;
        a->extradata_size = i->extradata_size;
    }
    avio_open(&o_fmt_ctx->pb, filename, AVIO_FLAG_WRITE);//打开输出文件
    avformat_write_header(o_fmt_ctx, NULL);//写文件头
    _isStartWriteFile = YES;
}

2.写包：

    AVPacket packet;
    BOOL finished = NO;
    while (!finished) {
        if (av_read_frame(_formatCtx, &packet) < 0) {
            _isEOF = YES;
            break;
        }
        if (packet.size > 0 && _isStartWriteFile == YES) {
            for (int i=0; i<packet.size; i++) {
                if (packet.data[i]== (0x00)&&packet.data[i+1]==0x00&&packet.data[i+2]==0x00&&packet.data[i+3]==0x01&&((packet.data[i+4])&0x1f)==5) {//i帧
                    isIFrame = YES;
                }
            }
            if (isIFrame==YES) {
                AVPacket i_pkt;
                av_init_packet(&i_pkt);
                av_copy_packet(&i_pkt, &packet);
                //这里要重新计算pts、dts
                if (packet.stream_index==0) {
                    if (isvYES) {
                        i_pkt.pts -= lastv_pts;
                        i_pkt.dts -= lastv_dts;
                    }
                    else {
                        lastv_pts = packet.pts;
                        lastv_dts = packet.dts;
                        i_pkt.pts = packet.pts-lastv_pts;
                        i_pkt.dts = packet.dts-lastv_dts;
                        isvYES = YES;
                    }
                }
                else if (packet.stream_index==1) {
                    if (isaYES) {
                        i_pkt.pts -= lasta_pts;
                        i_pkt.dts -= lasta_dts;
                    }
                    else {
                        lasta_pts = packet.pts;
                        lasta_dts = packet.dts;
                        i_pkt.pts = packet.pts-lasta_pts;
                        i_pkt.dts = packet.dts-lasta_dts;
                        isaYES = YES;
                    }
                }
                av_interleaved_write_frame(o_fmt_ctx, &i_pkt);//写包
            }
        }
        av_free_packet(&packet);
    }

3.写尾：

- (void)stopRecordAction {
    if (_isStartWriteFile == YES) {
        av_write_trailer(o_fmt_ctx);//写文件尾
        _isStartWriteFile = NO;
    }
}